Parotid mass: Pleomorphic adenoma (mixed tumor).

Some 60 to 70% of parotid masses are pleomorphic adenomas. These benign lesions are well marginated. They can have a fibrous capsule that contains myoepithelial cells, occasionally with foci of capsular penetration, or an incomplete fibrous capsule. They are usually heterogeneous to varying degrees on magnetic resonance imaging (MRI) and contrast-enhanced on computed tomography (CT).

A small percentage of pleomorphic adenomas do appear to be homogeneous (figure 1). On MRI, these tumors reflect a low signal on T1 and a bright signal on T2. They are usually round or oval; otherwise, they can be lobulated, with sharply defined borders. Pleomorphic adenomas occur more often in the superficial portion of the parotid gland. Calcifications occasionally occur within the lesion. When the tumor arises from the deep portion of the gland, it can extend into the parapharyngeal area; therefore, it should be distinguished from other parapharyngeal masses.

Pleomorphic adenomas of the superficial portion of the gland are usually removed by superficial parotidectomy, after the branches of the facial nerve have been identified. Failure to make clear resection margins increases the likelihood of recurrence.

Recurrent pleomorphic adenomas are often seen as clusters of multiple nodules close to the previous scar and facial nerve. These recurrent nodules are best seen on contrast-enhanced T2 MRI (figure 2). Uncommon malignant lesions associated with pleomorphic adenomas are carcinoma, malignant mixed tumors, and metastasizing mixed tumors. These unusual malignancies appear to occur in patients who have had a mass for a long time that suddenly increased in size.

From the Department of Radiology, MacNeal Hospital, Berwyn, Ill.

Reference

(1.) Carter BL. Salivary glands. In: Imaging of the Head and Neck. Valvassori GE, Mafee MF, Carter BL, eds. New York: Thieme Medical Publishers, 1995:475-509.